(1) Field of the Invention
The present invention relates to microwave generation and transmission systems, and more particularly to a modular microwave source comprising a novel Blumlein-like microwave oscillator and integrated antenna.
(2) Description of the Related Art
High Power Electromagnetic (HPEM) systems are being investigated for potential use for disabling motor vehicles, vessels, and aircraft for security and law enforcement purposes. It has been proposed that properly configured and directed microwave radiation can induce currents in wiring of a vehicle that can cause a malfunction of the vehicle's electronic control module (“ECM”) and/or other electronic components that can cause the vehicle's engine to stop running, thereby disabling the vehicle. Being able to remotely disable a vehicle would be useful, for example, in perimeter protection of installations and facilities as well as in law enforcement activities such as car chases.
Inducing a disabling current in vehicle wiring typically requires that wiring connected to the vehicle's ECM be subjected to an electromagnetic field having a frequency, energy density, and duration sufficient to induce sufficient current to cause the ECM to malfunction. At the same time, the electromagnetic field must not be harmful to human occupants of the vehicle at which it is directed.
Prior art HPEM systems can be widely grouped into three categories based on the system bandwidth: narrowband, mesoband, and ultra-wideband. The technologies of these classes of sources are quite different. Narrowband HPEM systems use expensive and bulky pulsed power systems to deliver energy to a relativistic electron beam. This beam is made to interact with a microwave cavity where kinetic energy from the beam is converted into microwave energy. These systems have extremely high average powers, but are very costly, power hungry, and inefficient. They are viable for platforms with significant power and weight budgets, such as ships, large airplanes, military ground vehicles, etc. Ultra-wideband (UWB) systems are at the opposite end of the spectrum. These systems have peak powers comparable to narrowband systems, but generate shorter-duration waveforms, thereby having much lower average power. These systems have the advantage of coupling across decades of bandwidths, but at very low energy spectral densities.
Mesoband systems have recently emerged as an outgrowth of wideband technology. Mesoband HPEM systems typically use components and techniques that were originally designed for UWB applications and couple them with a microwave oscillator to produce a waveform that has much lower bandwidth than UWB systems. Mesoband systems produce higher average power over a narrower bandwidth than UWB systems, giving significantly higher power spectral densities than similar UWB systems. The most important parameter in both the Mesoband and UWB systems used to direct energy in a specific direction is the maximum electric field produced at the radiating aperture. Present systems are limited to values much less than the breakdown electric field of the atmosphere.
Components of an example prior art Mesoband system are shown in FIG. 1. In the example shown in FIG. 1, the components include a primary power supply 110, a power conditioner 120, a pulse generator 130, an oscillator 140, and an antenna 150. Primary power supply 110 is an available AC or DC power source. Power conditioner 120 rectifies, accumulates and/or multiplies the voltage supplied by primary power supply 110. Power conditioner 120 may, for example, comprise one or more capacitors that are charged by power supply 110 in parallel and then discharged either in parallel or in series. A Marx generator is a well known power conditioner used in HPEM systems that multiplies the voltage of a power supply by charging capacitors in parallel and then discharging them in series.
Transmission line pulse forming networks (“PFNs”) may be used in Mesoband systems as pulse generators and oscillators. An example of a basic transmission line PFN is shown in FIG. 2. The transmission line PFN of FIG. 2 comprises a transmission line 220 of length L and having impedance Z0. At one end, the two conductors of the line 220 are connected to an output load 230, such as, for example an antenna. Output load 230 has an impedance ZL. At the other end, the two conductors are connected to a high-speed switch 240 (such as, for example, a spark gap) and a high voltage power source 210. To generate high frequency microwaves, high speed switch 240 should be able to operate at high voltages and high currents with sub-nanosecond resistive transition time and minimum inductance. Transmission line 220 is charged by power source to voltage V0 (typically at 10-50 KV for HPEM system applications). When high speed switch 240 is closed, transmission line 220 discharges through output load 230. If the impedance of the output load 230 is matched to the impedance of the transmission line 220 (i.e. ZL=Z0), the transmission line operates as a pulse generator and the output is a single pulse of voltage 0.5 V0 with a duration dependent on the length L of transmission line 220. If the impedances are mismatched (i.e. ZL≠Z0), the transmission line operates as an oscillator and the output is a damped sinusoid whose frequency depends on length L.
FIG. 3 shows a variation of the transmission line PFN of FIG. 2 called a “Blumlein transmission line pulse generator” or simply a “Blumlein.” A Blumlein uses two transmission lines to create an output pulse of voltage V0, twice the voltage of the pulse produced by a single transmission line PFN.
The Blumlein of FIG. 3 includes transmission lines 320 and 350, having lengths L1 and L2, and impedances Z1 and Z2, respectively. The inner conductors of transmission lines 320 and 350 are connected together, while the outer conductors are connected across output load 330. In the Blumlein Pulse system of FIG. 3, a high speed switch 340 and high voltage power source 310 are connected between the inner and outer conductors of line 320. To generate a pulse using the Blumlein of FIG. 3, transmission line 320 is charged to voltage V0 by power source 310, and then discharged by closing high-speed switch 340. If the two lines 320 and 350 have equal lengths and impedances, and if the output load impedance is equal to the sum of the line impedances, then the output is a single pulse of voltage V0 with a length dependent on the lengths of lines 320 and 350. If the lengths and impedances of the two lines are equal, but the load impedance does not match the line impedances, the output is a damped sinusoid whose frequency depends on the length of the two lines. If the lengths and impedances of the two lines are not equal and the load impedance is not matched to the line impedances, the output is a combination of two damped sinusoids whose frequencies depend on the respective lengths of the two lines.
Another configuration of a prior art Blumlein, referred to as a “tri-plate Blumlein,” is shown in FIG. 4. In the tri-plate Blumlein shown in FIG. 4, metal plates are used in place of the transmission lines of the Blumlein of FIG. 3. Center plate 450 takes the place of the inner conductors of lines 320 and 350 in FIG. 3. Top plate 460 takes the place of the outer conductor of line 350 of FIG. 3, and bottom plate takes the place of the outer conductor of line 320 in FIG. 3. As shown in FIG. 4, a high voltage power supply 410 charges center plate 450 with respect to bottom plate 420 A high speed switch 440 is connected between bottom plate 420 and center plate 450. In operation, high voltage power conditioner 120 is used to charge center plate 450. After the plate is charged, closing high speed switch 440 causes a high-voltage pulse to be generated in the similar manner as in the transmission line Blumlein of FIG. 3.
Antennas commonly used in HPEM systems include, for example, parallel plate waveguides and horn and helical antennas.
To be practical for mobile uses (such as being mounted on cars or small aircraft such as police helicopters), a vehicle disabling HPEM system must be small enough to be mountable to the vehicle platform (e.g. police car or helicopter), must be tunable over a wide frequency range (e.g. 200-1350 MHz), and must be able to deliver rapid and repeated bursts of directed microwave energy. Prior art HPEM systems are not able to meet this need.